1. Field of the Invention
The present invention relates to the installation of cables. The invention is primarily concerned with fibre optic communication cables but may be applied to other cables, for example co-axial or other communication or power cables. The invention is particularly concerned with providing a cable connection over an extended span, particularly over a sea or other water crossing and in a preferred application is concerned with providing a deep sea (for example transatlantic) cable crossing; however, aspects of the invention may be applied to land based or other applications.
2. Background Art
There are a variety of known techniques for laying cables, whether sub-sea or otherwise, each of which has associated problems.
Long distance sea-crossing cables, for example current transatlantic sub-sea cables, are often deployed by unreeling armoured cable from a cable carrying vessel. There are known techniques for suspending the cable at a distance below the surface, for burying cables in shallow waters and for armouring the cables to withstand various attacks. However, installation of such cables remains expensive and problematic and the cables are susceptible to damage.
Provision of a cable link over an extended span, particularly a sea crossing, is a major undertaking and it is normal practice to plan each link to provide excess cable capacity at the time the link is constructed to accommodate future traffic for a given period until the next cable link is planned; this reduces repetition of the expensive time-consuming and hazardous work involved in installing a cable on the sea bed. The intention is normally that, once laid, the cable(s) will require minimal maintenance or disturbance underwater. The landing stations are well defined, growth in demand can be predicted, and it is likely that the cable will have a finite life so will need to replaced at some point. Therefore, despite the large cost of creating a link, it is usual to provide a link in the expectation that a new link will be needed in a few years. Indeed, because of the risk of damage to an individual link, it is considered desirable that new links over new routes are added over time.
By way of additional background, in urban cable networks, in contrast to point-to-point long-distance links, it is likely that new individual connections will be needed frequently, but the precise routing and connection points cannot necessarily be predicted. The urban environment is, of course, far more accessible and less hostile than the sea bed. It is therefore known to provide multiple empty ducts under roadways with regular access points, enabling new cables to be pulled through the relatively short distances between access points when a new connection is required. Installation of the ducts is, in comparison to installation of sub sea cables, a relatively straightforward task, involving merely digging up a section of roadway and is routinely performed by general road construction contractors; the techniques employed on such projects would not normally be considered applicable to the engineering challenges of a major sub sea link.